The present invention relates to a method for producing fluoronitrobenzene compounds.
Fluoronitrobenzene compounds such as 2-fluoronitrobenzene, 4-fluoronitrobenzene, and 2,4-difluoronitrobenzene, are useful as intermediates for the synthesis of various herbicidal compounds, dyes, and the like. Such compounds have been prepared from corresponding chloronitrobenzene compounds by so-called halogen exchange reactions, illustrated as follows: ##STR1## wherein MF represents an alkali metal fluoride salt. The reaction is generally conducted in an aprotic, polar, organic solvent, such as dimethylsulfoxide, dimethylformamide, tetramethylenesulfone (sulfolane), and the like.
Alkali metal fluoride salts are not soluble in such solvents. Therefore, the reaction mixtures usually contain two phases, i.e., solid and liquid phases or two immiscible liquid phases. Finger, et al., J. Am. Chem Soc., 78, 6034 (1956) and Duesel, et al., U.S. Pat. No. 3,064,058 (1962), describe the reaction of chloronitrobenzene compounds with finely-divided, solid potassium fluoride in aprotic polar solvents to produce corresponding fluoronitrobenzene compounds. Boudakian, et al., U.S. Pat. No. 3,240,824 (1966), describe the reaction of o-chloronitrobenzene with solid potassium fluoride at elevated temperatures, without any solvent or diluents, to produce o-fluoronitrobenzene. Napier and Starks, U.S. Pat. No. 3,992,432 (1976), describe a reaction involving two liquid phases. In the Napier and Starks reaction, the inorganic fluoride salt is dissolved in an aqueous phase, and the chloronitrobenzene compound is dissolved in a water-immiscible, organic phase. The reaction is catalyzed by a quaternary salt, which reportedly transfers ions across the phase interface.
Use of quaternary salt phase-transfer catalysts in solid-liquid, two phase reactions also has been known. For instance, Kunz, U.S. Pat. No. 4,069,262 (1978), describes the production of 2-fluoronitrobenzene by reacting 2-chloronitrobenzene with ultrafine particulate potassium fluoride in tetramethylenesulfone solvent using a macrocyclic ether (crown ether) or a quaternary ammonium halide (such as tetrabutylammonium chloride, benzyltrimethylammonium chloride, benzyltrimethylammonium fluoride or benzyltriethylammonium chloride) as the catalyst.
Tull, et al., U.S. Pat. No. 4,140,719 ( 1979), describes the production of 2,4-difluoro-5-chloronitrobenzene by reacting 2,4,5-trichloronitrobenzene with a solid fluorinating agent selected from NaF, KF, CsF, and C.sub.1-4 alkyl quaternary ammonium fluoride, and mixtures thereof under substantially anhydrous conditions in the presence of a quaternary compound solid-liquid phase transfer catalyst. The liquid phase comprises an organic solvent in which the trichloro compound is soluble and the fluorinating agent is essentially insoluble. It is disclosed that, for example, the quaternary catalyst compound must be soluble in toluene and accordingly the four radicals in the catalyst compound must be selected so that the total number of carbon atoms is at least 18. In a list of quaternary compounds which may be employed as transfer catalysts, the total number of carbon atoms ranges from 26 (for trioctylethylammonium bromide) to 38 (for dioctadecyldimethylammonium chloride).
Starks, "Selecting a Phase Transfer Catalyst," Chemtech (Feb. 1980), pages 110-117, describes patterns that purportedly enable prediction of catalysts for anion transfer from aqueous or solid inorganic phases to organic phases. Starks mentions "the normally used 14-30 carbon" catalyst cations (p. 114) and refers to the cations (C.sub.8 H.sub.17).sub.3 NCH.sub.3.sup.+, (C.sub.4 H.sub.9).sub.4 N.sup.+, C.sub.16 H.sub.33 P(C.sub.4 H.sub.9).sub.3.sup.+, (C.sub.6 H.sub.5 CH.sub.2 N(C.sub.2 H.sub.5).sub.34.sup.+ and (C.sub.16 H.sub.33)N(CH.sub.3).sub.3.sup.+ as those most frequently used in quaternary salt catalysis (p. 110). According to Starks, "In order to have anion transfer, the phase transfer cation plus the desired anion must be substantially partitioned into the organic phase" (p. 113) and a quaternary cation having few carbon atoms "usually will not be sufficiently lipophilic to effect transfer, unless the organic phase is highly polar and the anion to be transferred has a substantial organic structure" (p. 114).
North, U.S. Pat. No. 4,287,374 (1981) discloses a process for the production of a monofluoronitrobenzene in which a monochloronitrobenzene is heated with an alkali metal fluoride and a phase transfer catalyst at a temperature of no more than 200.degree. C., preferably 125.degree.-170.degree. C., especially 140.degree.-150.degree. C. North discloses, as examples of such catalysts which may be used, long chain alkylammonium halides, e.g., tetradecyltrimethylammonium bromide (which has 17 carbon atoms), aralkylammonium compounds, e.g., benzyltriethylammonium chloride or hydroxide (which have 13 carbon atoms) and alkylphosphonium halides, e.g., hexadecyltributylphosphonium bromide (which has 28 carbon atoms). Organic polar aprotic solvents, e.g., dimethylsulfoxide, dimethylformamide or sulpholane, reportedly can be used.
In general, halide-exchange reactions for preparing fluoronitrobenzene compounds by reacting chloronitrobenzene compounds with fluoride salts in aprotic, polar organic solvents in the presence of quaternary ammonium salt phase-transfer catalysts proceed at faster rates when conducted at elevated temperature relative to rates obtainable at lower temperature. However, quaternary ammonium phase-transfer catalysts employed in heretofore known methods are less stable at higher temperature and have been found to decompose or lose their catalytic activity at elevated reaction temperatures. Moreover, U.S. Pat. No. 4,418,229 (to White), incorporated herein by reference, discloses that lower molecular weight catalysts, i.e., those having a total number of carbon atoms less than about 16, are less stable under the conditions (including elevated temperature) of the method of the invention disclosed therein than the therein preferred catalysts of higher molecular weight having about 16 or more carbon atoms. Quaternary ammonium salt phase transfer catalysts such as tetrabutylammonium halides, e.g., (C.sub.4 H.sub.9).sub.4 NBr and (C.sub.4 H.sub.9).sub.4 NCl, cetyltrimethylammonium halides, e.g., C.sub.16 H.sub.33 (CH.sub.3).sub.3 NBr and C.sub.16 H.sub.33 (CH.sub.3).sub.3 NCl, and Aliquat.RTM. 336 (available from McKerson Corp., Minneapolis, Minn. and designated chemically as tricaprylmethylammonium chloride) have been found to decompose rapidly at elevated temperature (e.g., 170.degree.-175.degree. C.) with concomitant rapid substantial depletion of catalytic activity (e.g., in less than one half hour).
The above cited White patent discloses the finding that in the conversion of chloronitrobenzene compounds to corresponding fluoronitrobenzene compounds using a quaternary ammonium salt phase-transfer catalyst at elevated temperatures, a high level of catalytic activity can be maintained by adding the catalyst to the reaction mixture incrementally during the course of the reaction.